1. Field of the Invention
The present invention relates to a communication circuit and a vehicle communication apparatus using the same, and more particularly, to a communication circuit capable of securely starting transmission of information to thus perform communication in a short time while reducing the influence by environmental noise, and a vehicle communication apparatus using the same.
2. Description of the Related Art
In a related art, in a wireless or wired communication system or remote control system, a signal has been used to control start of transmission of information. For example, a transmission side attaches a start bit to a head of an information bit string that is divided into predetermined lengths and transmits the information, and a reception side can read out the information by using the start bit. Such asynchronous communication has been widely performed. The length of the bit string of the information is not limited to a length corresponding to one character and includes an appropriate number of bits depending on purposes.
In various communications, the transmission is performed by modulating carrier waves with the signal of the bit string of the information. For short-distance transmission of relatively small information, a frequency shift keying or amplitude shift keying is used because transmission reception circuits can be implemented easily and inexpensively. In case of the amplitude shift keying of changing an amplitude of the carrier wave, the carrier wave having a single frequency is modulated in a time-division manner by the signal including the start bit and the information bit string, so that the transmission is performed.
The above communication system is also widely used for communication between in-vehicle devices that are mounted on a vehicle. For example, a communication system is known, which transmits operation information from a passenger to a reception apparatus from a console or remote controller so as to control the various in-vehicle devices by an operation switch that is provided to the console or the various in-vehicle devices by an operation switch that is provided to the console or remote controller. In this vehicle communication system, when the wireless communication is performed using radio signals, the communication is interfered by electromagnetic noises that are generated from the in-vehicle devices or vehicle surroundings. When it is difficult to securely recognize the start bit due to the noises, a transmission error is caused. A method is also performed, in which a specific signal is transmitted ahead of the start bit so as to enable the reception side to securely recognize the start bit.
As a vehicle communication system which transmits data having a frame structure, a communication system is disclosed in which a plurality of channels having different frequencies is used to transmit a data frame through a first channel and then to transmit a data frame that is extended by adding dummy bits to data through a second channel, so that the wireless communication is performed even under noise environments that interfere the reception of the radio signals of the first channel (see JP-A-2009-278597).
As described above, in the transmission by the frequency shift keying or amplitude shift keying of a single carrier wave, the start bit to give a start timing of the transmission and the information bit string are transmitted in a time division manner. Therefore, when it is difficult for the reception side to recognize the start bit to give the start timing of the transmission due to the exogenous noises and the like, the reception error is caused. In addition, when the start bit and the bit string of information are indistinguishable, the reception error is caused. As the measures, a method has been suggested in which an extra signal is added or a start bit is made to be redundant. However, a problem is caused in that the communication time is prolonged. In addition, when the reception error is caused, the retransmission is required to recover the error, so that the communication is further prolonged.
In particular, in the vehicle communication system under noise environments, a communication method is demanded which enables the communication in a shortest time by using a simple transmission and reception circuit. Recently, a system that transmits the information so as to control the various in-vehicle devices has been rapidly increased. The related-art communication methods that use radio signals, infrared and wires have a problem in that the configuration or control of the communication
In addition, the operation and input devices for controlling the in-vehicle devices have been also larger and complicated. A plurality of operation switches is arranged around the console, controller or seat of a vehicle, so that it is difficult for a user to operate the switches. Therefore, a vehicle communication apparatus is demanded which can make the operation and input devices smaller and reduce the number of operation switches by using a simple communication circuit.